Lightweight boron carbide materials with improved mechanical properties and process for their manufacture

ABSTRACT

This disclosure describes sintered bodies comprising about 90 wt % to about 99 wt % of boron carbide, having a B:C atomic ratio ranging from 3.8 to 4.5:1; 0 to 1 wt % free carbon; 0 to 1 wt % BN or AlN, remainder an oxide binder phase; said sintered body having a uniform microstructure composed of substantially equiaxed grains of said boron carbide; the oxide binder phase comprising at least a rare earth aluminate and optionally Al 2 O 3  or other ternary or binary phases of rare earth oxide-alumina systems; the binder phase being present in form of pockets at the multiple grain junctions and the density of no more than 2.6 g/cm 3 . Also described is a manufacturing process for the above described substantially pore-free, sintered boron carbide materials with high strength and fracture toughness, which can be used for production of large-area parts. This is achieved by liquid phase low temperature-low pressure hot pressing of boron carbide in an argon atmosphere.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to substantially pore-free boron carbidesintered bodies with a density of not more than 2.60 g/cm³ and improvedmechanical properties, and to a process for their manufacture.

2. Background Art

Boron carbide, B₄C, is a lightweight solid (density, 2.52 g/cm³) thathas high hardness and a high resistance to abrasive wear and has beenused mainly as an abrasive. In dense, sintered form it has been appliedas armor for bulletproof body vests, for vehicles and aircraft, as wearresistant linings such as sand blasting nozzles and as control rods innuclear reactors.

Densification of boron carbide to relative densities of above 95% TD(theoretical density) typically requires small additions of amorphouscarbon as a sintering aid and takes place at temperatures of at least2100° C. Nevertheless for full densification (>99% TD) a hot pressingtreatment is required. Boron carbide has the disadvantage of highbrittleness, i.e. monolithic boron carbide ceramics have a very lowfracture toughness which varies between 2.1 and 2.6 MPa·m^(1/2).

Self-bonded boron carbide has therefore not hitherto become establishedin applications as a structural ceramic, where strong and toughcomponents are required. Fracture toughness of boron carbide armorshould be improved, since according to LaSalvia [Ceram. Sci. and Eng.Proceedings 23, 213-220 (2002)] the armor ceramics should have both highhardness and high toughness to prevent penetration of the projectile.

Attempts have therefore been made to reinforce boron carbide, like otherbrittle monolithic ceramics, by dispersion of particulate hardmaterials. Thus, U.S. Pat. No. 5,543,370 to Sigl et al., for examplediscloses the toughening of boron carbide by the addition of titaniumdiboride (TiB₂) and free carbon. The solid state sintered and HIPpost-densified composites (HIP-conditions 2100° C., 200 MPa argonpressure) with 20 and 40 vol-% TiB₂ exceed both the toughness and alsothe strength of pure boron carbide, with four-point flexural strengthvalues in the range of from 550 to 740 MPa and K_(IC) values in therange of from 4.7 to 6.8 MPa·m^(1/2). The sintered densities of theseB₄C based composites varied significantly with the amount of TiB₂, i.e.2.90 g/cm³ for 20 vol-% TiB₂ and 3.30 g/cm³ for 40 vol-% TiB₂. In asimilar way to titanium diboride, other borides of the transition metalsof the groups 4a to 6a of the Periodic Table, in equilibrium with B₄Ccan also be used to improve the mechanical properties of boron carbide.EP 1,452,509 Al to Hirao et al., discloses a boron carbide chromiumdiboride (CrB₂) composite, sintered at 2030° C. containing a dispersionof 10 to 25 mol-% CrB₂ particles, and having a 4-point flexural strengthin the range of 436 to 528 MPa and a fracture toughness of at least 3.0MPa·m^(1/2), respectively. While the above mentioned composite materialsare formed by mixing the desired metal boride phases and subsequentsintering, processes have also been described in which the desired metalboride composition is only formed after a suitable reaction of thestarting materials during the sintering step. Skorokhod and Krstic[“High Strength—High Toughness B₄C—TiB₂ Composites”, J. Mater. Sci.Lett., 19, 237-239 (2000)] have successfully fabricated a 85 B₄C-15 TiB₂(vol-%) composite with a flexural strength of 621 MPa and a fracturetoughness of 6.1 MPa·m^(1/2) (measured by the SENB method with a 100 μmnotch width) by reaction hot pressing of a sub-micron particle sizedboron carbide powder using additions of sub-micron size TiO₂ and carbonat a temperature of 2000° C. The formation of uniform distributed TiB₂particles (<5 μm grain size) was in accordance with the reaction(1+x)B₄C+2 TiO₂+3 C→xB₄C+2 TiB₂+4 CO

The high strength of this material was attributed to the combination ofhigh fracture toughness and fine microstructure. A further improvementof the mechanical properties of 80 B₄C—20 TiB₂ (mol-%) composites isdisclosed in EP 1,452,509 A1 to Hirao et al., wherein via use ofnanometer size TiO₂ powder, carbon black and sub-micron particle sizedB₄C powder after reaction hot pressing at 2000° C. with a very highpressure of 50 MPa, dense sintered bodies (density 2.82 g/cm³, 100% TD)with both high flexural strength (720-870 MPa) and high toughness (2.8to 3.4 MPa·m^(1/2), SEPB method) could be obtained. The improvement ofmechanical properties was attributed to the fine-grained microstructureand uniform dispersion of TiB₂ particles.

However, the proposed toughening method by dispersion of metal borideshas technological disadvantages in view of the used densificationprocesses (1) and concerning other material properties of the densifiedend product (2).

(1) Densification Processes

Solid state sintering of B₄C—TiB₂ composites requires high sinteringtemperatures in the range of from 2000 to 2175° C. Sintering of B₄C—CrB₂composites is possible at 2030° C., however densification is incomplete(residual porosity above 2%). Via reaction hot pressing at 2000° C. 100%dense B₄C—TiB₂ composites can be obtained, however, the high moldingpressure of 50 MPa used (see [0056] in EP 1,452,508 to Hirao et al.)restricts hot pressing to small area parts. Moreover, homogeneousdistribution of ultra-fine TiO₂ and carbon black in a methanol-B₄Cdispersion and drying of the flammable slurry are delicate processes anddifficult to scale-up.

(2) Other Material Properties of Densified End Product

Since for toughening by particle dispersion the optimum volume contentof added or in-situ grown particles is relatively high, densities ofcomposites were increased, e.g., to 2.82 g/cm³ for a 15 vol-% TiB₂ andto 3.32 g/cm³ for a 40 vol-% TiB₂ composite, respectively. However, forlightweight armor application the density should remain as low aspossible (below 2.60 g/cm³). Further, since hardness of TiB₂ issignificantly lower (comparable only to SiC), the resulting hardness ofthe B₄C—TiB₂ composites is inferior to the commercial grades ofmonolithic B₄C ceramics. Therefore, the relatively high densitiescombined with a lower hardness inhibits the use of tough B₄C—TiB₂composites as a lightweight ceramic armor material.

Another approach to produce tough and high strength B₄C ceramics is touse liquid phase sintering. Lee and Kim [J. Mat. Sci. 27 (1992),6335-6340] have shown that the addition of alumina, Al₂O₃, promoted thedensification of boron carbide and a maximum density of 96% oftheoretical can be obtained with 3 wt-% alumina-doped B₄C sintered at2150° C., i.e. above the melting point of Al₂O₃. The microstructureshowed equiaxed B₄C grains with a mean grain size of about 7 μm.However, as the addition of Al₂O₃ exceeded 3 wt-% exaggerated graingrowth occurred, which was attributed to the liquid phase.

It has been reported by Kim et al., [J. Am. Ceram. Soc. 83, No. 11,2863-65 (2000)], that by hot pressing of B₄C with alumina additions upto 5 vol-% at 2000° C. the mechanical properties can be remarkablyincreased as compared to undoped, hot pressed B₄C of 88% relativedensity. Fracture toughness increased steadily with the addition ofAl₂O₃ from ˜3 MPa·m^(1/2) (2.5 vol % Al₂O₃) up to 3.8 MPa·m^(1/2) (10vol-% Al₂O₃). However the achieved flexural strength was below 560 MPa.

The use of yttria (Y₂O₃) containing sintering aids was first describedin two Japanese Patent Applications, JP 62012663 to Kani (pressurelesssintering of B₄C with mixed additions of 4 wt-% Al+1 wt-% Si+3 wt-% Y₂O₃at 2000° C.) and JP 08012434 to Kani (pressureless sintering with 0.5wt-% Al+3 wt-% Y₂O₃ at 2000° C.). It was shown that instead of Y₂O₃ onecan also use other oxides, nitrides, carbides or borides, the net resultbeing the same. However, these processes are complicated due tosintering in atmospheres containing high aluminum partial pressures.Furthermore, no improvement in fracture toughness of B₄C materials wasreported.

The possibility to improve fracture toughness of boron carbide withyttria or mixed additions of yttria in combination with other oxides wasfirst demonstrated in U.S. Pat. No. 5,330,942 to Holcombe et al., and CN1,438,201 to Li et al., respectively.

According to the method with is disclosed in U.S. Pat. No. 5,330,942,the fracture toughness of B₄C can be increased to 3.9 MPa·m^(1/2) byvacuum sintering at 1900 to 1975° C. using powder compacts ofcomposition 97.5 B₄C-2.5 carbon (wt-%) packed in a yttria grit of 0.15to 1.4 mm grain size. The vacuum allows yttrium oxide vapor to penetratethe powder compact promoting reaction-sintering of carbon-doped B₄C tofull density (2.62 g/cm³). The final composite showed an overall yttriumcontent of 9.4 wt-%, the yttrium being present in the form of Y—B—O—Ccontaining 5 μm particulates dispersed evenly in a matrix of 40 μm boroncarbide grains. X-ray diffraction identified that yttrium boride andyttrium borocarbide coexist with B₄C. However, owing to uncontrolled gasinfiltration this method is as yet unsuitable for mass production ofliquid phase sintered B₄C.

The Chinese patent application CN 1,438,201 also discloses a method toincrease the toughness of boron carbide while maintaining a reasonablehardness and intermediate strength. The basis of the method is to useliquid phase sintering under vacuum or streaming argon of powdercompacts comprising B₄C powder (average particle size 0.6 to 3.5 μm) and1 to 28 wt-% additions of Y₂O₃ in combination with Al₂O₃ or aluminumnitride (AlN) and any one of La₂O₃ or CeO₂ components. The B₄C materialof example 4 (starting from a sub-micron powder mixture 95.2 B₄C-0.8La₂O₃-1.7 AlN-2.3 Y₂O₃, wt-%) obtained by pressureless sintering at1920° C. for 270 minutes (i.e. 4.5 hours hold at max. temperature) wascharacterized with regard to mechanical properties: Vickers hardness2950, four-point bend strength 520 MPa and fracture toughness 5.4MPa·m^(1/2). No indication of the microstructure and the chemicalcomposition of the final B₄C sintered bodies was given. However, withthe proposed method of pressureless sintering, in particular in view ofthe used atmosphere (vacuum/streaming argon gas), the long hold times attemperatures of around 1900° C., it is necessary to consider reactionsbetween boron carbide and the rare earth oxides of the liquid phase.Reactions accordingY₂O₃+3 B₄C→2 YB₆+3 CO andLa₂O₃+3 B₄C→2 LaB₆+3 COresult in formation of rare earth borides, in weight losses (evolutionof carbon monoxide) and in a decrease in sintered density of the bodies.Due to this, production of large parts using this process would bedifficult to control. This is supported by experiments of the presentinventors who, while reproducing the example 4 of CN 1,438,201, obtainedvery porous bodies (sintered density of only 2.05 g/cm³, correspondingto 80% of theoretical density). The obtained samples were found tocontain rare earth hexaboride (LaB₆) and rare earth borocarbide (YB₂C₂)in addition to boron carbide. Thus, due to the decomposition reactionsthe liquid phase was depleted to such an extent that is was not possibleto make dense bodies.

The present invention differs from the teachings of U.S. Pat. No.5,330,942 and CN 1,438,201 relating to (1) a low-temperaturelow-pressure hot-pressing densification without any appreciable reactionbetween B₄C and the liquid phase, (2) a new B₄C material containing arare earth aluminate as main component of the oxide binder phase, andhaving unique mechanical properties.

SUMMARY OF THE INVENITON

It is one object of the present invention to overcome the abovedescribed drawbacks of liquid phase sintered B₄C bodies and to providesubstantially pore-free sintered B₄C materials which can be used astough structural ceramics and lightweight armor ceramics which exhibithigh strength, toughness and hardness. This objective is achieved bysintered bodies comprising about 90% to about 99% by weight of boroncarbide, having a B:C atomic ratio ranging from 3.8 to 4.5:1; 0 to 1% byweight free carbon; 0 to 1% by weight BN or AlN, and remainder an oxidebinder phase; said sintered body having a uniform microstructurecomposed of substantially equiaxed grains of said boron carbide; theoxide binder phase comprising at least a rare earth aluminate andoptionally other ternary or binary phases of rare earth oxide—aluminasystems; the binder phase being present in form of pockets at themultiple grain junctions. The sintered bodies can contain small amounts(total less than 2% by weight) of non-oxide impurities like free carbon(carbon non-covalently bound to boron in B₄C), boron nitride oraluminium nitride, which are present as a result of the process or asresidue in the powder mixture used as starting material. In the contextof the present invention rare earth (RE) is to be understood as meaningthe metals Sc, Y, the lanthanides and actinides. Preferably, the rareearth metal is yttrium.

In one embodiment of the material of the present invention, the oxidebinder phase comprises two yttrium aluminates, YAlO₃ and Y₃Al₅O₁₂. Thesintered body has the following properties: density of not more than2.60 g/cm³, the boron carbide grains having a mean grain intercept sizebetween about 5 to about 12 μm, porosity not more than 1.0%, hardness(Knoop, 300 g load) greater than 2400, 4-point flexural strength in therange of about 400 to about 600 MPa, and fracture toughness of at least3.0 MPa·m^(1/2) (measured by the Chevron Notch method).

In a preferred embodiment of the material of the present invention, theoxide binder phase comprises yttrium aluminate Y₃Al₅O₁₂ and alumina from0 to 50% by weight based on the total binder phase content. The sinteredbody has the following properties: density of not more than 2.60 g/cm³,the B₄C grains having a mean grain intercept size ranging from 0.7 toabout 3 μm, porosity below 0.5%, hardness (HK0.3) greater than 2500,4-point flexural strength greater than 700 MPa, and fracture toughnessgreater than 3.5 MPa·m^(1/2) (measured by the Chevron Notch method).

It is another object of the present invention to provide a process formanufacture of the above described substantially pore-free sinteredboron carbide materials capable also of production of large-area parts.This object is achieved by liquid phase hot pressing of boron carbidecomprising forming a powder mixture comprising boron carbide having aB:C atomic ratio in the range of 3.8 to 4.5:1 and up to at most 10% byweight of a sintering aid selected from the group of RE-aluminates,mixtures of at least one RE-aluminate with alumina or AlN, and mixturesof a rare earth oxide with alumina or AlN; and hot pressing the powdermixture under a die pressure of below 15 MPa at a temperature belowapproximately 2000° C. in an argon atmosphere (so-called “lowtemperature-low pressure hot pressing method”). In a preferredembodiment of the process of the present invention the powder mixturecomprises boron carbide having a mean particle size of about 0.4 toabout 1.0 μm and up to at most 10% by weight of a sintering aid selectedfrom the group of yttrium aluminates (Y₃Al₅O₁₂, YAlO₃, Y₄Al₂O₉),mixtures of at least one yttrium aluminate with alumina or AlN, andmixtures of yttria with alumina or AlN, and hot pressing the powdermixture under a die pressure of from 6.9 to 13.8 MPa (1000-2000 psi) ata temperature of below approximately 1850° C. in an argon atmosphere.

Teachings of the present invention show that, unexpectedly, by lowpressure-low temperature hot pressing boron carbide using theyttria-alumina system (forming a liquid phase), with the proportion ofalumina in the mixture being from 30 to 60% by weight of alumina, notonly sub-micron particle sized B₄C powders but also low-cost, relativelycoarse-grained B₄C powders can be densified without exaggerated graingrowth to virtually theoretical density. The B₄C bodies so produced showboth high fracture toughness and hardness but exhibit lower strengthrelative to the finer grained powder, due to their coarsermicrostructure. The strength of the material of this embodiment isnevertheless higher than the state of the art hot pressed boron carbide.Therefore in another embodiment of the process of the present inventionthe powder mixture comprises boron carbide having a mean particle sizeof about 4 to about 8 μm and up to at most 10% by weight of a sinteringaid selected from the group of yttrium aluminates (Y₃Al₅O₁₂, YAlO₃,Y₄Al₂O₉), mixtures of at least one yttrium aluminate with alumina orAlN, and mixtures of yttria with alumina or AlN, and hot pressing thepowder mixture under a die pressure of from 6.9 to 10.3 MPa (1000-1500psi) at a temperature of below 2000° C. in an argon atmosphere.

As compared with the existing state of the art in hot pressing of B₄Cusing graphite tooling and high temperatures of at least 2000° C.,typically over 2100° C., production costs for the material can beconsiderably reduced. High temperatures and pressures often lead todamage of the graphite tooling in form of edge fracture due to reactionswith the densified material and due to high temperature deformation ofthe tooling. The process of the present invention considerably reducesthe reaction between the densified material and the graphite tooling, aswell as the tooling deformation.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects and advantages of the present invention, aswell as additional objects and advantages thereof, will be more fullyunderstood hereinafter as a result of a detailed description of apreferred embodiment when taken in conjunction with the followingdrawings in which:

FIG. 1 is a field-emission SEM-micrograph (magnification 10 000×)showing the substantially pore-free microstructure of the preferredmaterial of the present invention, prepared by hot pressing at 1825° C.using a sub-micron particle sized B₄C powder (mean grain size ˜0.6 μm)and 7.6% by weight sintering additions based on a mixture of 68 wt-%Y₃Al₅O₁₂ and 32 wt-% AlN; and

FIG. 2 is a field-emission SEM-micrograph (magnification 3000×) showingthe substantially pore-free microstructure of a sintered body of thepresent invention, prepared by hot pressing at 1925° C. using arelatively coarse-grained B₄C powder (mean grain size, ˜6 μm) and 6.6%by weight sintering additions based on a 60 wt-% yttria-40 wt-% aluminamixture.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In liquid phase sintering it is desirable to have a liquid that wets thematrix grains and allows particle rearrangement to occur in the presenceof the liquid phase. The liquid phase sintering aids and the ratio ofliquid phase sintering aids may be selected based on the melting pointsof the rare earth aluminates or the liquid temperatures for given ratiosof RE₂O₃ to alumina and RE₂O₃ to AlN in the systems RE₂O₃—Al₂O₃ andRE₂O₃—AlN, respectively. The role of aluminum nitride, AlN, as acomponent in the sintering additions is partly seen in its reaction withboron oxide accordingB₂O₃+2 AlN→2 BN+Al₂O₃when oxygen-rich boron carbide starting powders are used. Further it isknown that aluminum nitride contributes to liquid formation through thepresence of eutectics in the systems AlN—RE₂O₃.

Preferably the sintering additive used is a mixture of AlN with acompound from the group consisting of RE₂O₃ or RE-aluminates in powderform as grain fractions of 10 μm and finer, preferably 7 μm and finer,with mean particle sizes in the range of 1 to 2 μm. The proportion ofAlN in the mixture with the RE₂O₃ additive corresponding to an RE₂O₃:AlNmolar ratio in the range from 1:4 to 1:14, and in the mixture with anRE-aluminate to at least 10 to 50% by weight AlN. Particularlypreferable for liquid phase sintering of coarse-grained B₄C powders, amixture of yttria and alumina is used, the proportion of alumina in themixture with yttria is from 30 to 60% by weight of alumina,corresponding to melting temperatures between 1940 and 1760° C.

The amount of sintering additions should be in the range of from about1.0 to about 10% by weight. With amounts below about 1.0% by weightsufficient densification cannot be achieved. In order to achieve highhardness in combination with the other properties the amount ofsintering additions should not exceed 10% and preferably 8.5% by weight.Preferably an amount of sintering additions within the range of from 2.5to 7.5% by weight, is used.

For carrying out the process of the invention it is preferable to use asa starting material boron carbide powders with a purity of at least 97%by weight, which means that the sum of the boron content and the carboncontent should be at least a total of 97% by weight, at a B:C atomicratio of 3.8 to 4.5:1. Metallic impurities can be tolerated up to atotal of 0.5% by weight. The remaining impurities, up to 100% by weight,are distributed among oxygen and nitrogen in form of adherent boronoxide and boron nitride. Typical powders have adherent boron oxidecontents less than 2.0% by weight and nitrogen contents less than 0.5%by weight, respectively. As a measure of the required particle fineness,use is advantageously made of the average particle size (measured bylaser diffraction) and the specific surface area (measured by theBET-method). Boron carbide powders can be used with an average particlesize in the range of 0.3 to 8 μm and a specific surface area in therange of 1 to 25 m²/g, respectively.

The blending of boron carbide, sintering additives and optionallyorganic binders for producing pre-shaped parts is affected in a mannerknown per se, for example by means of dry homogenisation or bydispersion in a liquid forming a slurry which is subsequently dried(e.g., spray-dried or similar). Suitable organic binders can be added tothe powder mix to facilitate shape forming. The dried powder mix or thepre-shaped part is then placed into a graphite hot press die.

If pre-shaped parts are employed, the organic binders can be removed ina separate furnace step at temperatures in the range between 100 and600° C. in an inert atmosphere or air, or they can be removed during thehot pressing step.

After filling the molds the temperature is elevated by heating anddepending on the B₄C powder grain size, the size of the furnace and theparts, the densification is accomplished at temperatures between 1800and 1950° C. while applying a relatively low pressure of between 6.9 and13.8 MPa.

During cooling of the hot pressed bodies, the liquid phase solidifiesforming a solid, crystalline binder phase, consisting of at least oneRE-aluminate (e.g., Y₃Al₅O₁₂) and optionally α-Al₂O₃, which ispreferentially formed at the multiple junctions between the boroncarbide crystals.

The hot pressing is performed in an inert atmosphere such as argon orhelium. Temperatures of about 1950° C. and higher and pressures higherthan 15 MPa do not yield further increase in density. Processing timesabove the liquidus temperature should be limited so as to inhibit graingrowth. The times should be less than about 3 hours, with time at themaximum sintering temperature preferably less than two hours (dependingon the load size).

The substantially pore free sintered boron carbide bodies producedaccording to the invention are characterized by the combination of thefollowing properties:

-   1. Sintered density of not more than 2.60 g/cm³-   2. Residual porosity of not more than 1% (based on density)-   3. High hardness demonstrated by Knoop-300 g values of from 2400 to    2700, i.e. hardness is almost equal or higher as given in the    literature for single-phase dense B₄C [A. Lipp et al.: “Hardness and    hardness determination of non-metallic hard materials, I. Boron    Carbide”, Ber. Dtsch. Keram. Ges. 52, No. 11, 335-338 (1975), in    German]-   4. High four-point flexural strength in the range of from 400 to 820    MPa·m^(1/2)-   5. High fracture toughness demonstrated by K_(IC) values of from 3.0    to 4.0 MPa·m^(1/2), according to the Chevron Notch method.

The materials have, in addition, a microstructure consisting ofsubstantially equiaxed boron carbide grains having a mean intercept sizeof from about 0.7 to about 12 μm and a homogeneously distributed oxidebinder phase visible at the multiple grain junctions in accumulationspredominantly smaller than the B₄C grains in diameter. The presence ofrare earth aluminates and optionally alumina as crystalline compounds inthe oxide binder phase is detectable using SEM and EDX micro analysis(materials with binder phase contents below ˜5% by weight) as well as byX-ray powder diffraction analysis (materials with binder phase contentsof more than 5% by weight).

The materials of the present invention are suitable in particular forthe manufacture of armor plates, of wear resistant components such astools for the cutting machining of non-ferrous metals, wood, plastic andceramic green bodies, and air jet nozzles and other structuralcomponents that require hardness, high strength and toughness.

The invention is further clarified by the following examples. Examples 1through 10 show the low-temperature low-pressure hot pressing ofsub-micron particle sized B₄C together with properties of final B₄Cmaterials. Examples 11 through 18 show that liquid phase hot pressing ofrelatively coarse grained B₄C starting powders lead to B₄C materials ofimproved mechanical properties.

Testing Procedures

The densities were determined using the Archimedes technique in water.The relative density, in % TD (Theoretical Density), is based on thetheoretical density of the B₄C hot pressed materials.

The residual porosity (P) was calculated from the relative densityaccording to% P=100−% TD.

The flexural tests were conducted on 3×4×50 mm test bars with surfacesground according to ASTM C1161 (Method B). Flexural strength values(σ_(B), 4-pt) were typically mean values of 10 or more measurements.

To determine the fracture toughness of the B₄C materials the ChevronNotch method was used (ASTM C1421).

The Knoop Hardness (HK0.3), was measured (5-10 indentations per sample)with a Knoop diamond at a load of 300 g.

For micro-structural characterization, samples were polished to 1 μmdiamond finish and etched with diluted sulfuric acid or by electrolyticetching. The intercept method was employed to measure the mean interceptgrain size.

EXAMPLES 1-10

A sub-micron grade B₄C powder 1 μm and finer, with an average particlesize of 0.6 μm was mixed in a water suspension with the sinteringadditives by stirring.

The B₄C powder had a specific surface area of 17.6 m²/g and had thefollowing chemical composition:

B total 76.7 wt-% C total 20.4 wt-% C free 0.8 wt-% B₂O₃ 1.5 wt-% N 0.3wt-% Si 400 ppm Fe 500 ppm Al 100 ppm Ti 600 ppm Ca 10 ppm(taking into account the free carbon and the boron present as B₂O₃ andthe BN, corresponds to a B:C atomic ratio of this powder is 4.3).

Sintering additives used were: fine aluminum nitride (AlN) powder incombination with fine yttrium aluminate, (Y₃Al₅O₁₂); AlN in combinationwith yttria; alumina in combination with yttria; and Y₃Al₅O₁₂ incombination with alumina. The sintering additives had an averageparticle size finer than 10 μm and a specific surface area of 2-7 m²/g.Aqueous slips were prepared in accordance with the formulations given inTable 1, with variation of the type and the quantity of the sinteringadditives within the limits of 0 to 7.6% by weight added sintering aids,6 parts by weight of organic pressing aids also being used per 100 partsby weight B₄C plus sintering additive (=doped sintering powder). Thehomogenized slips with a 60% powder concentration were dried by means ofa spray drier. Free flowing, pressable granules were obtained (meangranule diameter 50-70 μm) with a residual moisture content ofapproximately 0.3% by weight. Billets with dimensions 4.1×4.1 inch weredry pressed in a steel die (˜1 inch thick) from each composition.

Formed parts were loaded in graphite hot press dies, and the parts werehot pressed in an argon atmosphere with the maximum temperatures andpressures being 1825° C. and 10.3 Mpa, respectively. Examples 1-8 showconditions of the present teachings. One formulation with Y₃Al₅O₁₂—AlNsintering aids was hot pressed at 2000° C. (instead of 1825° C.) withidentical pressure load of 10.3 MPa to show influence of a too highsintering temperature on the B₄C material properties.

Also, the sub-micron B₄C powder without sintering additions was hotpressed using as sintering parameters 1825° C./10.3 MPa (Example 9) and2100° C./30.9 MPa (Example 10), demonstrating that high temperature andhigh pressure are necessary for full densification without sinteringaids.

TABLE 1 Composition of the powder batches (% by weight) Hot presstemperature Sintering (° C.)/pressure Example No. B₄C* Y₂O₃ YAG** AlNAl₂O₃ aids, total (MPa)  1 92.39 — 5.16 2.45 — 7.6 1825/10.3  2 96.18 —2.59 1.23 — 3.8 ”  3 92.96 2.24 — 4.80 — 7.0 ”  4 96.48 1.12 — 2.40 —3.5 ”  5 95.00 2.00 — — 3.00 5.0 ”  6 97.50 — 2.00 — 0.50 2.5 ”  7 98.000.80 — — 1.20 2.0 ”  8 92.39 — 5.16 2.45 — 7.6 2000/10.3  9 100 — — — —0.0 1825/10.3 (prior art) 10 100 — — — — 0.0 2100/30.9 (prior art)*contains 0.8% free C and 1.5% adherent B₂O₃ **Y₃Al₅O₁₂ (Y-Al-Garnet)

After cooling down, the parts were removed from the dies and cleaned.The parts separated easily from the graphite dies with exception of partof Example 10 (adhered to the die). Density, porosity, flexuralstrength, fracture toughness, hardness and grain size were measured.Table 2 gives the hot pressed body properties.

The data shows that the process according to the invention can becarried out with a mixture of Y₃Al₅O₁₂ with AlN, mixtures of yttria withAlN, mixtures of yttria with alumina, and a mixture of Y₃Al₅O₁₂ withalumina as sintering aid, the net result being similar. The Examples 1-7show that the low temperature—low pressure hot pressed B₄C bodiespossess a density of below 2.60 g/cm³, a relative density of more than99.5% TD (corresponds to a residual porosity of below 0.5%), a flexuralstrength of at least 700 MPa, a fracture toughness of at least 3.5MPa·m^(1/2), and a hardness of at least 2500.

TABLE 2 Hot pressed body properties Mean Relative Flexural FractureGrain Example Density Density Residual Strength Toughness Hardness Size,No. g/cm³ % TD Porosity % MPa MPa · m^(1/2) HK-0.3 kg/mm² μm 1 2.59 99.80.2 762 3.8 2510 1.2 2 2.55 99.7 0.3 751 3.6 2650 1.6 3 2.58 100 0.0 8193.9 2520 0.9 4 2.54 99.7 0.3 713 3.6 2620 1.7 5 2.58 100 0.0 802 3.72560 1.4 6 2.55 100 0.0 722 3.6 2680 1.9 7 2.53 99.6 0.4 704 3.9 26702.1 8 2.59 99.8 0.2 485 3.1 2550 10.5 9 2.23 88.5 11.5 — — — — 10 2.5099.2 0.8 526 2.4 2500 4.1

The B₄C material of Example 8 was sufficiently densified but strengthwas at least 30% lower as compared to materials of Examples 1-7. It isapparent from Example 8 that, starting from a sub-micron B₄C powder andliquid phase hot pressing at a temperature of more than about 1950° C.,strength is reduced because a coarse-grained microstructure is obtainedat such higher temperatures. To anyone familiar with the art, this showsthat by optimizing the additives, starting powders and hot pressingconditions, one can optimize the material properties and processmanufacturing costs depending on the needs.

It is apparent from Example 9 that a B₄C material with open porosity isobtained by hot pressing (1825° C./10.3 MPa) without using a liquidphase. On the other hand Example 10 shows that the B₄C material obtainedby hot pressing without a liquid phase but using enhanced sinteringconditions (2100° C./30.9 MPa) was of course sufficiently densified buthad a poor fracture toughness of self-bonded B₄C (2.4 MPa·m^(1/2))

FIG. 1 shows the extremely fine-grained microstructure of the B4Cmaterial from Example 1 which was hot pressed at 1825° C./10.3 MPa with7.6% by weight sintering additives based on a Y₃Al₅O₁₂—AlN mixture. TheB₄C grains (grayish phase) have an equiaxed grain form, a mean grainsize of 1.2 μm, and the binder phase (light phase) occurs atinter-granular regions between the B₄C grains. Densification to residualporosities below 0.5% at a temperature of 1825° C. and a low-pressureload of 10.3 MPa (1500 psi) can be attributed to a liquid phase near theY₃Al₅O₁₂—Al₂O₃ eutectic composition (70 wt-% Y₃Al₅O₁₂-30 wt-% Al₂O₃,eutectic temperature 1760° C.). By wet chemical analysis of the B4Cmaterial from Example 1, an elemental composition of 71.3 B-20.3 C-0.6N-2.9 O-2.65 Al-2.19 Y (wt-%) was determined. By X-ray diffractionanalysis the yttrium aluminate Y₃Al₅O₁₂ and Al₂O₃ with very littlegraphite and boron nitride were identified as secondary phasescoexistent with boron carbide.

EXAMPLES 11-19

A standard grade B₄C powder 20 μm and finer with a mean grain size of 6μm was mixed dry with a “master mix”—a previously slurry homogenized anddried boron carbide, alumina powder (specific surface area 5 m²/g) and ayttria powder (specific surface area 5 m²/g). Also an aqueous slip ofthis B₄C powder with Y₃Al₅O₁₂ powder (specific surface area 4 m²/g)alone was prepared.

The B₄C powder had a specific surface area of 2.5 m²/g and had thefollowing chemical composition:

B total 77.5 wt-% C total 21.2 wt-% C free 0.9 wt-% B₂O₃ 0.5 wt-% N 0.3wt-% Si 300 ppm Fe 700 ppm Al 100 ppm Ti 200 ppm Ca 30 ppm(which, taking into account the free carbon and the boron present asB₂O₃ and the BN, corresponds to a B:C atomic ratio of 4.2).

Total quantity of sintering additives was within the limits 2.0 to 8.3%by weight. The formulations mixed were (A) 2 wt-% Al₂O₃, (B) 5 wt-%Y₂O₃-3.3 wt-% Al₂O₃, (C) 4 wt-% Y₂O₃-2.6 wt-% Al₂O₃, (D) 3 wt-% Y₂O₃-2wt-% Al₂O₃, (E) 2 wt-% Y₂O₃-1.3 wt-% Al₂O₃, and (F) 5 wt-% Y₃Al₅O₁₂.

Binder was added to the powder and the powder was dried, and screened.Billets with dimensions 4.1×4.1 inch were dry pressed in a steel die (˜1inch thick) from each composition. Parts were loaded in graphite hotpress dies, and the parts were hot pressed in an argon atmosphere.Compositions (A) through (F) were densified using 1925° C./10.3 MPa ashot pressing parameters (Examples 10-16); in addition, compositions (D)and (E) used lowered hot pressing parameters 1900° C./8.3 MPa (Examples17 and 18).

After cooling down, the parts were removed from the dies and cleaned.The parts separated easily from the graphite dies.

Density, porosity, strength, toughness, hardness and grain size wasmeasured.

The properties data of the hot pressed B4C bodies according to Examples11-18 are given in Table 3 and compared to commercially produced B₄Cmaterial (Example 19).

Data of Example 19 correspond to a commercial, self-bonded B₄C material,hot pressed using prior art sintering aids.

TABLE 3 Hot pressed body properties Example Relative Flexural FractureMean No. Density Density Residual Strength Toughness Hardness Grain(Composition) g/cm³ % TD Porosity % MPa MPa · m^(1/2) HK-0.3 kg/mm²Size, μm 11* 2.26 84.0 16.0 — — — — (A) 12 2.59 99.0 1.0 480 3.4 24109.4 (B) 13 2.58 99.4 0.6 451 3.4 2580 9.1 (C) 14 2.56 99.3 0.7 417 3.32520 9.9 (D) 15 2.54 99.3 0.7 421 3.0 2470 10.3 (E) 16 2.56 99.3 0.7 4113.3 2500 10.0 (F) 17 2.57 99.7 0.3 436 3.4 2510 9.8 (D) 18 2.54 99.3 0.7401 3.1 2500 11.1 (E) 19** <2.52 >98.5 <1.5 330 2.2-2.6 2490 10-40 *forcomparison (using 2% by weight alumina) **for comparison (commercial B₄Cmaterial, prior art)

It is apparent from Example 11 that insufficient densification, i.e. aB₄C material with open porosity, is obtained using low-temperaturelow-pressure hot pressing of coarse-grained B₄C powder (6 μm, mean grainsize) with alumina as sintering aid alone. The Examples 11-18 show thatby low temperature—low pressure hot pressing of coarse B₄C withsintering additives based on yttria—alumina mixtures or Y₃Al₅O₁₂ alone,B₄C materials can be produced which possess a density of below 2.60g/cm³, a relative density of at least 99.0% TD (corresponds to aresidual porosity of not more than 1.0%), a flexural strength of atleast 400 MPa, a fracture toughness of at least 3.0 Mpa·m^(1/2), and ahardness of at least 2400.

FIG. 2 shows the microstructure of the B₄C material from Example 13which was hot pressed at 1925° C./10.3 MPa with 6.6% by weight sinteringadditives based on an yttria—alumina mixture. The B₄C grains (grayishphase) have an equiaxed grain form, a mean grain size of 9.1 μm, and thebinder phase (white) occurs as intergranular inclusions between the B₄Cgrains. The good densification to residual porosities of not more than1.0% at temperatures of 1900-1925° C. and low pressure loads can beattributed to a liquid phase near the yttrium aluminate composition (60wt-% Y₂O₃-40 wt-% Al₂O₃, liquation above 1870° C.).

By wet chemical analysis of the B₄C material from Example 13 anelemental composition of 73.6 B-20.9 C-0.23 N-1.9 0-1.21 Al-2.20 Y(wt-%) was determined. By X-ray diffraction analysis the two yttriumaluminates YAlO₃ and Y₃Al₅O₁₂ with very little graphite and boronnitride were identified as secondary phases co-existent with boroncarbide.

While several embodiments of the present invention have been shown anddescribed, it is to be understood that many changes and modificationsmay be made without departing from the spirit and scope of the inventionas defined in the appended claims.

1. A substantially pore-free, sintered body comprising from about 90% byweight to about 99% by weight of boron carbide, having a B:C atomicratio ranging from 3.8 to 4.5:1, 0 to 1% by weight free carbon, 0 to 1%by weight BN or AlN, remainder an oxide binder phase; said sintered bodyhaving a density of not more than 2.60 g/cm³, a substantially uniformmicrostructure composed of substantially, equiaxed grains of said boroncarbide, having a mean grain size within the range of from 0.7 to about12 μm; the oxide binder phase comprising at least a rare earthaluminate; the binder phase being present in a form of accumulations atmultiple grain junctions, said sintered body having a porosity of notmore than 1%, hardness (Knoop, 300 g) greater than 2400; four-pointflexural strength of at least 400 MPa; and fracture toughness of atleast 3.0 MPa·m^(1/2) measured by the Chevron Notch method.
 2. Thesubstantially pore-free sintered body of claim 1 wherein said oxidebinder phase is at least one yttrium aluminate selected from the groupY₃Al₅O₁₂, YAlO₃, and Y₄Al₂O₉.
 3. The substantially pore-free sinteredbody of claim 1 having the following properties: density of not morethan 2.60 g/cm³, the boron carbide grains having a mean grain size fromabout 5 to about 12 μm, porosity of not more than 1%, hardness (HK0.3)greater than 2400; four-point flexural strength of at least 400 MPa; andfracture toughness of at least 3.0 MPa·m^(1/2) measured by the ChevronNotch method.
 4. The substantially pore-free sintered body of claim 1having the following properties: density of not more than 2.60 g/cm³,the boron carbide grains having a mean grain size from about 0.7 toabout 3 μm, porosity of not more than 0.5%, hardness (HK0.3) of at least2500; four-point flexural strength of at least 700 MPa; and fracturetoughness of at least 3.5 MPa·m^(1/2) measured by the Chevron Notchmethod.